(in other words: this topic is NOT about Xavier, Wolverine, Magneto, Cable, Wolverine, Cyclops, Shadowcat, Wolverine etc.
Were talking characters hardly being used.
With room for characters that Always appeared, but never really delved into.

1) Killed of second appearance.
Editor (Always the editor ) considered him too similar to Wolverine.

60s had an annoying character unwilling to fit in, Mimic.
So the Professor kicked him from the team after just two issues.
Could have done with Thunderbird. Still available for future stories.

2) Could write an entire essay on him.
But even on Generation X, he and Emma argued so much that neither of them became plausible as teacher.
And then the character assassination in the end (alcoholic; X-corps)

3) On the series since 1963.
Apart from Louise Simonson, no one gave him more credit than being immature.

There have been others that I may name in a later post, but dear god Neophyte was fantastic, he explored and weighed the differences and similarities of the dreams of Magneto and Xavier like no character had ever done before or since, he was in a great position and I totally wanted to see more of him.

1) Killed of second appearance.
Editor (Always the editor ) considered him too similar to Wolverine.

60s had an annoying character unwilling to fit in, Mimic.
So the Professor kicked him from the team after just two issues.
Could have done with Thunderbird. Still available for future stories.

2) Could write an entire essay on him.
But even on Generation X, he and Emma argued so much that neither of them became plausible as teacher.
And then the character assassination in the end (alcoholic; X-corps)

3) On the series since 1963.
Apart from Louise Simonson, no one gave him more credit than being immature.

1) The truth is Proudstar was always intended to be killed off no matter what to add a sense of Drama to the new era. Anything could happen and he was supposed to die in Giant-Sized X-Men in his first appearance but the editor convinced the writers to hold back to the regular series started up again. The similarities to Wolverine were because Thunderbird was a character that nothing was ever going to be done with so why put effort into making him stand out? That was all on writer Len Wein. This is why Thunderbird was written as bitter and angry with a need to prove himself in those two stories. He was going to get himself killed with that attitude.

3) Actually around OZT when a lot of veterans left the team Iceman stepped up and was portrayed as a mature, capable leader. It pops up every so often when smaller ad-hoc squads are sent out. Iceman is relied on to be the one who "brings the kids home in one piece" and has shown in smaller one-off issues that a lot of his immaturity rep is because he sees himself as "the Original Kid Member" and takes on the burden of being the class clown to break up the tension so the other veteran members keep their perspective while worrying about the weight of the world.

The truth is Proudstar was always intended to be killed off no matter what to add a sense of Drama to the new era. Anything could happen and he was supposed to die in Giant-Sized X-Men in his first appearance but the editor convinced the writers to hold back to the regular series started up again. The similarities to Wolverine were because Thunderbird was a character that nothing was ever going to be done with so why put effort into making him stand out? That was all on writer Len Wein. This is why Thunderbird was written as bitter and angry with a need to prove himself in those two stories. He was going to get himself killed with that attitude.

Is that “the truth” though? I don’t recall ever seeing that attested in an interview by someone with actual knowledge. IIRC, Claremont said he killed off Thunderbird because he was redundant and CC didn’t know what to do with him. But I’ve never heard it said that was Wein’s intention for including him.

EDIT: From Secrets of the X-Men:

"When we were first planning out that first issue, we decided what we were going to do was have it be an aptitude test or an entrance exam or something like that," Cockrum revealed in The X-Men Companion. "They would be sent off to rescue the original X-Men, but the original X-Men would not actually be in any danger. We figured if it's an entrance exam, theoretically, there are people who are going to flunk as well as people who pass, and so we had Banshee and Sunfire, and we were gonna flunk 'em. Then, we thought, "Well, that doesn't seem fair, we ought to have a new guy to flunk too, a new guy who's unsuitable." So that was what Thunderbird was for, to be a flunker. He was unsuitable because he was anti-social."

"But at the last minute well, I liked Banshee and we all liked Thunderbird, so we figured to hell with it. It turned out not to be a test anyway. So we had Sunfire, who nobody much liked, go off in a huff, and we kept Banshee and we kept Thunderbird. But then we didn't know what to do with Thunderbird because we never thought him out. It was easier to kill him off than to think him out."

So, sort of both. According to Cockrum, they really didn’t have a clear plan for Thunderbird.

Funny you mention Mimic, Gurney, because he's my number one pick for underused character with great potential. All of the X-Men's powers plus the ability to copy others, thats powerful. I like what they've dome with his personality over the years, giving him anxiety and doubting himself (that sounds so cruel I know). It makes him more interesting than your usual cocky confident hero type

Rusty and Skids, for sure. I feel like they could have been the Scott & Jean of New Mutants.
Marrow, MAGGOTT, and Dr. Reyes are long-time faves for me.
I've been glad to see him in Generation X v2, but Chamber.
The Acolytes. Given that they were meant to be the X-Men of Magneto, the fact that almost NONE of them got personalities or backgrounds breaks my heart. We always got ALMOST tastes, but then they'd drop off the map for like eight months.

Official member of the Mighty Avengers fan club. "You want Avengers, lady? Take a good look. We're the Avengers."

The Acolytes. Given that they were meant to be the X-Men of Magneto, the fact that almost NONE of them got personalities or backgrounds breaks my heart. We always got ALMOST tastes, but then they'd drop off the map for like eight months.

You are totally right about the Acolytes. I always thought there was wasted potential there. I would love a mini set in the past that shows the Acolytes forming and descending into Fabian Cortez's crazed cult.

working on my list I saw some noteworthy overlap with grief's, here goes nothing

MARROW
Whilst it was interesting so see a Morlock joining the X-Men proper, her biggest selling point for me was that she was SUCH a unique take on the "young girl of the team" role. Before her Shadowcat and Jubilee idolized Wolverine, then Marrow came and slit his throat. That and combining the "young girl" and "disfigured mutant" niche in 1 character was unheard of too. Bravo.

AMELIA VOGHT
Grief mentioned all of the Acolytes, and he wasn't wrong. I singled out Neophyte but man I can single out this one too. Her arguments with Xavier were strong stuff, as was her role among the Acolytes. Give me more of her.

ROCKSLIDE & ANOLE
This one is simple, their little team-up as the X-Dudes was the most fun I had reading an X-story this century. Give me more of it.

BISHOP
Now this one is interesting, as he has seen far more usage than the other characters I mention. This for me has to do with the fact that new characters just don't stick, and for years I considered Bishop the most recently introduced x-character that was gonna stick, but by now I conclude Bishop did miss out on that level I thought he was gonna reach, and I do find that a pity.

Acolytes kinda like alot of non-stars of villain teams were very underdeveloped...but it happens to villains who come out as apart of a team vs villains who come out as individuals and then form teams. The one X-ception is probably Toad. Although he doesn't get "shine" now (I guess depends on how you define it), he's had a long and storied history since the '60s. And even now he still gets talking parts in a number of stories.

Whereas all my favorite Acolytes (the Kleinstock Brothers, Chrome, Neophyte, and Gargouille) have had very little story time. The most used Acolytes are probably Scanner, Amelia and of course Cortez/Exodus.

There have been others that I may name in a later post, but dear god Neophyte was fantastic, he explored and weighed the differences and similarities of the dreams of Magneto and Xavier like no character had ever done before or since, he was in a great position and I totally wanted to see more of him.

(off-topic: Martijn, you happen to be Dutch too?
Or Flamish?)

Agreed.
Up untill now the only character who genuinely considered both visions.
All others were good guys who unfortunately got recruited by Magneto first (Wanda & Pietro) or only joined looking for answers to personal issues at the wrong place (Rusty & Skids, Colossus, Amelia Voight).

Which brings up another character who slipped through the cracks: SCANNER was shown in X-factor #92 (vol. 1) to start doubting Cortez as well and revealed being just a teenager in the X-men issue directly after Avalon got destroyed.

Why did Xavier start a school again? To prevent mutants from going a wrong direction the age they were most confused and impressionable, as teenagers.
Just acquiered the massachussets Academy, so plenty of room.
And with Emma Frost co-teacher of Generation X, Mystique and Sabretooth joining X-Factor, there was NO valid in-continuity reason for her not being admitted into Generation X.

I'd really like to see more of Frenzy, especially since her character was being developed following Age of X.

I'd also like to see Cipher. I understand she has similar powers to Kitty and with her being so prominent right now, it might be hard to include her. But then again, X-Men Gold has both Magma and the new Pyro which seems super redundant.

I don't get why Guggenheim keeps forcing Ink down our throats, when Cipher is a much more interesting creation of his.

Never worry what other people think of you, because no one ever thinks of you.

If Frenzy comes back I honestly wouldn't mind her with Adult Cyclops. It'd be interesting to see him with someone that's a bit more forceful, but at the same time not constantly scheming in some way or another. They were a nice couple in AoX.

I enjoy characters that are "untraceable" like Cipher seems to be. I'd like to see her in a book where stealth/subterfuge is the main goal of the team. Something similar to X-Force, but without the body count.

I enjoy characters that are "untraceable" like Cipher seems to be. I'd like to see her in a book where stealth/subterfuge is the main goal of the team. Something similar to X-Force, but without the body count.

Claremont had a clear and obvious trajectory for the character... well. For himself. It translated very poorly between his brain and the page.

Picture the X-Men as a group all operating out of the same place, or at the very least? With a summit of leaders of teams and factions on a regular basis.

Sage could easily be the Oracle of the X-Universe. Hell. even the MU. Play up her data processing powers and tone down her telepathy and jump-starting nonsense and she could totally assume the role of an information broker or central hub-based strategist, supplying the teams with intel or information pertinent to their mission.

EDIT FOR MORE

Whilst not strictly “underused” more “only used by one writer realistically”, i’d love to see more of Layla Miller. She’s been missing a while now and I’d love to see some interaction between her and some of the more spiritual X-Men, like Nightcrawler (who would have a very interesting perspective on her power since his return from death), Xorm (who has a spiritual element to his powers) and Armor (because her power comes from her ancestors and I kind of think that could be an interesting story).

I want to see more of the young X-Men too. Like... what have Surge et al been doing whilst they’ve basically been forgotten about?

And finally... whilst not strictly under-used, I kind of feel we need to see an interaction between Emma and the surviving Generation X kids...